Cambrian Stage Research Articles

A highly redox-heterogeneous ocean in South China during the early Cambrian (∼529–514 Ma): Implications for biota-environment co-evolution

Abstract The “Cambrian Explosion” is known for rapid increases in the morphological disparity and taxonomic diversity of metazoans. It has been widely proposed that this biological event was a consequence of oxygenation of the global ocean, but this hypothesis is still under debate. Here, we present high-resolution Fe–S–C–Al-trace element geochemical records from the Jinsha (outer shelf) and Weng'an (outer shelf) sections of the early Cambrian Yangtze Platform, integrating these results with previously published data from six correlative sections representing a range of water depths (Xiaotan, Shatan, Dingtai, Yangjiaping, Songtao, and Longbizui). The integrated iron chemistry and redox-sensitive trace element data suggest that euxinic mid-depth waters dynamically coexisted with oxic surface waters and ferruginous deep waters during the earliest Cambrian, but that stepwise expansion of oxic waters commenced during Cambrian Stage 3 ( ∼ 521 – 514 Ma ). Combined with data from lower Cambrian sections elsewhere, including Oman, Iran and Canada, we infer that the global ocean exhibited a high degree of redox heterogeneity during the early Cambrian, consistent with low atmospheric oxygen levels ( ∼ 10 – 40 % of present atmospheric level, or PAL). A large spatial gradient in pyrite sulfur isotopic compositions (δ34Spy), which vary from a mean of − 12.0 ‰ in nearshore areas to + 22.5 ‰ in distal deepwater sections in lower Cambrian marine units of South China imply low concentrations and spatial heterogeneity of seawater sulfate, which is consistent with a limited oceanic sulfate reservoir globally. By comparing our reconstructed redox chemistry with fossil records from the lower Cambrian of South China, we infer that a stepwise oxygenation of shelf and slope environments occurred concurrently with a gradual increase in ecosystem complexity. However, deep waters remained anoxic and ferruginous even as macrozooplankton and suspension-feeding mesozooplankton appeared during Cambrian Stage 3. These findings suggest that the “Cambrian Explosion” in South China may have been primarily a consequence of locally improved oxygenation of the ocean-surface layer rather than of the full global ocean. Our observations are inconsistent with predicted changes in ocean chemistry driven by early Cambrian animals, suggesting that the influence of early Cambrian animals on contemporaneous ocean chemistry, as proposed in previous studies, may be overly exaggerated.

Redox variations and organic matter accumulation on the Yangtze carbonate platform during Late Ediacaran–Early Cambrian: Constraints from petrology and geochemistry

In order to understand redox variations and organic matter accumulation on the Yangtze carbonate platform during the Late Ediacaran–Early Cambrian, petrological and geochemical studies of several wells were carried out in this work. Our data suggest that depositional environments were dominated by oxidizing bottom water conditions during the late Ediacaran, and evolved to anoxic conditions, triggered by blooms of microbial organisms in surface waters during the earliest Cambrian. Subsequently, massive release of H2S derived from both anaerobic recycling of organic matter and, probably, hydrothermal venting promoted a sulfidic ocean. The discovery of a Ni-Mo sulfide ore layer in the basal Cambrian implies that such a sulfidic condition spread onto the Yangtze carbonate platform interior during the late Cambrian Stage 2. Further, transgressive flooding led to widespread black shale deposition and persistently anoxic conditions, as indicated by geochemical proxies. During the late Ediacaran to earliest Cambrian, local and widespread phosphogenesis indicates that organic matter accumulation was intimately associated with microbial (especially cyanobacterial) blooms driven by phosphorus cycling. The organic matter accumulations in early Cambrian black shales, however, were in connection with anoxic bottom water conditions and intermittent replenishment of recycled organic phosphorus to surface waters.

The earliest pelagic jellyfish with rhopalia from Cambrian Chengjiang Lagerstätte

Modern cnidarian medusae generally show a triphasic life cycle with the succession of a larva, a sessile polyp and a pelagic medusa stage. The debate around the metagenesis of sessile polyps into pelagic medusae has lasted for more than 100years. When pelagic forms originated is not clear. Hitherto, the earliest crown-group medusae have been found at Cambrian Stage 5 (traditional Middle Cambrian, 509Ma) in Utah, while diverse stem-group medusozoans were found in the basal Cambrian Fortunian Stage. No reliable medusae have been found from Cambrian Series 2 Stage 3 (ca. 521Ma), although the marine benthic community teemed with many phyla of bilaterians, sponges and ctenophores. Here, we reinterpret Yunnanoascus haikouensisHu et al. (2007), originally described as a ctenophore, as a pelagic, predatory, crown-group medusozoan, based on the presence of rhopalia, possible radial canals and marginal tentacles. The medusae were a predatory member of the pelagic food web at the middle level of the ocean at Cambrian Stage 3.

Butterflies of the Cambrian benthos? Shield position in bradoriid arthropods

Mode of preservation and method of recovery strongly influences our understanding of the life habits of extinct organisms. Bradoriid arthropods were abundant, and diverse members of early Cambrian ecosystems and most life reconstructions display these animals with the two shields of the carapace open in a ‘butterfly’ configuration. This favoured reconstruction is largely based on the abundance of ‘crack-out’ specimens preserved in this position (e.g. Kunmingella from the early Cambrian of China). In contrast, large collections of acid processed bradoriids from the Arrowie Basin of South Australia (Cambrian Stage 3) are preserved with a narrow gape at the ventral margin or completely closed with the carapace folded along the dorsal midline. The relative abundance of conjoined, closed (or partially closed) specimens from the lower Cambrian Hawker Group succession suggests that at least some bradoriid taxa were capable of withdrawing appendages and tightly closing the shields, challenging the common view that the majority of bradoriids usually held their carapaces open in a ‘butterfly’ configuration during life. New data show that layers of the bradoriid carapace are continuous through the dorsal fold with no evidence for complex articulating structures as in ostracod hinges. The relatively pliable, sclerotized or lightly mineralized calcium phosphate composition of the carapace and the simple, flexible dorsal fold facilitated opening and closing of the shields. Despite not being closely related, ostracods share close biomechanical and ecological similarities with bradoriids. The evolution of more complex articulating hinge structures – together with well-developed musculature – in ostracods during the Early Ordovician, may have provided more efficient means for shield articulation and movement, thus promoting the ecological success of ostracods throughout the Phanerozoic.

Tectonostratigraphic history of the Ediacaran–Silurian Nanhua foreland basin in South China

This paper presents the tectonostratigraphic evolution of the Ediacaran–Silurian Nanhua Basin in South China and explores the relationship between clastic sedimentation in the basin and evolution of the adjacent Wuyi–Yunkai orogen. Sedimentary facies in the basin comprises, in an ascending order, turbiditic marine, shallow marine, and fluvial-dominated deltaic facies, featuring a lateral migration from southeast to northwest. We interpret the Ediacaran–Silurian Nanhua Basin as a foreland basin with a three-stage evolution history. Stage 1: the Ediacaran–Cambrian stage, recording the start of tectonic subsidence with turbiditic marine siliciclastic deposition, fed by exotic orogens outboard South China; Stage 2: the Ordovician to earliest-Silurian stage, characterized by a migrating depocenter with dominant shallow marine and deltaic siliciclastic deposition, fed by the local and northwestward propagating Wuyi–Yunkai orogen; Stage 3: the Silurian stage, showing the arrival of depocenter in the Yangtze Block during the waning stage of the orogeny with deltaic deposition in the remanent foreland basin. The Wuyi–Yunkai orogen remained the dominant sedimentary source region during Stage 3. Stage 1 was likely related to the collision of the South China Block toward northern India during the assembly of Gondwana, whereas Stages 2 and 3 recorded sedimentation during the northwestward propagation and subsequent orogenic root delamination/collapse of the Wuyi–Yunkai orogen, respectively. The Wuyi–Yunkai orogeny in South China is interpreted to have resulted from the far-field stress of the collision between South China and Indian Gondwana.

Upper Ovetian trilobites from Spain and their implications for the palaeobiogeography and correlation of the Cambrian Stage 3 in Gondwana

The upper part of the La Herrería Formation in Los Barrios de Luna (León Province, N Spain) has been revised from a palaeontological and biostratigraphical point of view. Two stratigraphic sections have been studied including their trilobite and ichnofossils contents. The ichnofossil assemblages have a high diversity of species characterising the Cruziana ichnofacies, suggesting a shallow sublittoral environment for the upper part of the La Herrería Formation. The trilobites species recognised are Lunagraulos antiquus, Dolerolenus formosus, Dolerolenus longioculatus, Lunolenus lunae, Metadoxides richterorum, Metadoxides armatus and Sardaspis? sp. from the upper Ovetian (lower Cambrian Stage 3 under discussion by the ISCS). The new trilobite assemblages make possible a good correlation between the lower Cambrian formations of North Spain, Sardinia, South China and Siberia. Analysis of the palaeobiogeographical meaning of all trilobite genera that have been identified in the upper Ovetian of Spain shows a strong connection between the northern peri-Gondwana margin and west Gondwana, with a low latitude distribution for the Spanish trilobites at this time.

Correlation of the Cambrian Evolutionary Radiation: geochronology, evolutionary stasis of earliest Cambrian (Terreneuvian) small shelly fossil (SSF) taxa, and chronostratigraphic significance

AbstractEarly faunas withWatsonella crosbyiwith or withoutAldanellaspp. have been equated with the Siberian Tommotian Stage (uppermost Terreneuvian) and used to define a proposed Cambrian Stage 2 base. Much earlier Terreneuvian occurrences are now shown by recovery of these micromolluscs below the I’ carbon excursion in the Siberian ‘Nemakit-Daldynian’ Stage and comparable δ13C excursions in the middle Meishucunian (China) and middle Chapel Island Formation (Avalonia). This δ13C excursion, a reliable Stage 2 marker, lies in ac. 10 Ma interval in the Cambrian Radiation in which long-ranged small shelly fossil taxa provide limited biostratigraphic resolution.

New Occurrences of Oldhamia in Eastern Yukon, Canada: Stratigraphic Context and Implications for Cambrian Deep-Marine Biostratigraphy

ABSTRACTFor over a century, the ichnogenus Oldhamia has been used as an index fossil for Cambrian deep-marine siliciclastic successions. New specimens from the Cambrian of Selwyn Basin, eastern Yukon, Canada, provide an opportunity to constrain the chronostratigraphic range of Oldhamia and test its potential for more detailed biozonation. The upper part of Narchilla Formation (Arrowhead Lake Member) yields O. antiqua, O. curvata, O. flabellata, and O. radiata. The overlying Gull Lake Formation yields a less diverse assemblage of O. antiqua, O. curvata, and O. flabellata. The upward disappearance of O. radiata is consistent with models that infer increasing complexity of Oldhamia ichnospecies through time but may also be a function of limited sampling. Regional stratigraphic correlations suggest Oldhamia appeared in Arrowhead Lake Member during Cambrian Stage 3. Archaeocyathans and trilobites from Gull Lake Formation confirm that Oldhamia was present during Cambrian Stages 3 to 4. A younger age limit on Oldhamia in Selwyn Basin is provided by syn-volcanic detrital zircons recovered from the base of the overlying Old Cabin Formation, which yield an age of 499.89 ± 0.14 Ma (earliest Guzhangian). Oldhamia appears to be a reliable index fossil for Cambrian Series 2 and Series 3 in Selwyn Basin.

Unlocking the early fossil record of the arthropod central nervous system.

Extant panarthropods (euarthropods, onychophorans and tardigrades) are hallmarked by stunning morphological and taxonomic diversity, but their central nervous systems (CNS) are relatively conserved. The timing of divergences of the ground pattern CNS organization of the major panarthropod clades has been poorly constrained because of a scarcity of data from their early fossil record. Although the CNS has been documented in three-dimensional detail in insects from Cenozoic ambers, it is widely assumed that these tissues are too prone to decay to withstand other styles of fossilization or geologically older preservation. However, Cambrian Burgess Shale-type compressions have emerged as sources of fossilized brains and nerve cords. CNS in these Cambrian fossils are preserved as carbon films or as iron oxides/hydroxides after pyrite in association with carbon. Experiments with carcasses compacted in fine-grained sediment depict preservation of neural tissue for a more prolonged temporal window than anticipated by decay experiments in other media. CNS and compound eye characters in exceptionally preserved Cambrian fossils predict divergences of the mandibulate and chelicerate ground patterns by Cambrian Stage 3 (ca 518 Ma), a dating that is compatible with molecular estimates for these splits.

The operculum and mode of life of the lower Cambrian hyolith Cupitheca from South Australia and North China

The operculum of the problematic tubular fossil Cupitheca holocyclata Bengtson in Bengtson et al., (1990) is described for the first time based on collections from South Australia and North China. The phosphatized sub-circular operculum exhibits well defined cardinal processes and a narrow cardinal shield unequivocally demonstrating that Cupitheca is a hyolith, probably an orthothecid. C. holocyclata has an almost global distribution in Cambrian Stages 3–4. The apical structure of the operculum is an elevated, disc-shaped platform with a concave base and a marginal rim that could represent the scar of a specialized larval attachment structure, perhaps anchoring the larval hyolith to a sediment grain, algae or other benthic substrate. Cupitheca probably had a pelagic larval stage and settled on the seafloor by attachment of the apical disc to suitable substrates before developing a free-living benthic adult lifestyle. This contrasting mode of life compared to other hyolith genera suggests that the group had already evolved a range of distinct lifestyles in the Cambrian, providing significant clues into their ecology and distribution.

Exceptionally preservedMickwitziafrom the Indian Springs Lagerstätte (Cambrian Stage 3), Nevada

AbstractExceptionally preserved specimens of the Cambrian stem-group brachiopodMickwitzia occidensWalcott, 1908 are described in detail from the Indian Springs Lagerstätte in Nevada, USA. Shell structure and preserved mantle setae from these specimens reveal a variable diagenetic (taphonomic) history and provide insight into the phylogenetic position of mickwitziids. Morphologic and morphometric comparison toM. monilifera(Linnarsson, 1869) from Sweden andM. muralensisWalcott, 1913 from British Columbia, Canada reveals clear species-level distinctions. Scanning electron microscopic analysis allows revision of the generic diagnosis. TheMickwitziashell is characterized by the presence of inwardly pointing phosphatic cones and tangential setae-bearing tubes. The inwardly pointing cone structures are not consistent with setal bearing structures as previously thought, but rather represent endopunctae-like structures. Acrotretid-like shell structures and shell-penetrating setae inM. occidensstrengthen the previously proposed close relationship between stem-group brachiopods and tommotiids, a group of small shelly fossils.

Reexamination of Yuknessia from the Cambrian of China and first report of Fuxianospira from North America

AbstractYuknessia Walcott, 1919 recently was transferred from the green algae to the Phylum Hemichordata on the basis of new details observed for the type species, Y. simplex, from the Burgess Shale Formation (Cambrian Stage 5) of British Columbia. This has prompted reexamination of material attributed to Yuknessia from various Cambrian localities in South China. Findings preclude both a Yuknessia and a hemichordate affinity for all of the Chinese study material, and most of this material is formally transferred to Fuxianospira Chen and Zhou, 1997, a taxon common in the Chengjiang biota. Comparable material from the Cambrian Marjum, Wheeler, and Burgess Shale formations of North America is also assigned to Fuxianospira, and this reassignment expands both the paleogeographic and stratigraphic range of this taxon. All aspects of the study specimens, including details obtained from scanning electron microscopy, are consistent with an algal affinity, as proposed in the original descriptions of the Chinese material.

The Cambrian brachiopod fauna from the first-trilobite age Shuijingtuo Formation in the Three Gorges area of China

The Yangtze platform of South China offers evidence within its Ediacaran–Cambrian geological record of the Cambrian explosion and diversification events in metazoan history. To understand the explosive radiation of animals and the environments in which it took place, the basal Cambrian fauna succession of the Aijiahe section in the Three Gorges area, western Hubei Province, has been studied, revealing the earliest brachiopod fauna (Tsunyidiscus trilobite Zone) in this region, which was dominated numerically by acrotretoids. This is accompanied by abundant skeletal fossils including minute well-preserved phosphatized archaeocyath cups and an assortment of abundant sponge spicules, chancelloriids, mollusks, hyoliths, and bradoriids, retrieved by acid-etching limestone interbeds in the black shale-dominated Shuijingtuo Formation (Series 2). The brachiopods comprise two species of acrotretoids, two types of botsfordiids (Botsfordiidae gen. et sp. indet. A and B), and four species of linguloids. Of the latter, Spinobolus popovi n. gen. n. sp. is strikingly distinctive and typified by spine-like ornamentation seen for the first time in the lower Cambrian; the remaining three linguloid genera, Palaeobolus, Eoobolus, and Lingulellotreta, have a trans-paleocontinental distribution. The Three Gorges Shuijingtuo brachiopod assemblage differs from that of the upper Atdabanian Stage (Cambrian Stage 3) in Siberia and South China, but shows great similarities with those discovered in the Tsanglangpuan (equivalent to Botoman or Stage 4) Stage of eastern Yunnan Province, Siberia, and South Australia, suggesting a much more prolonged sedimentary hiatus in basalmost Shuijingtuo Formation of the Three Gorges area than previously expected. The presence of such unconformities provides a caveat to stable isotope-based correlations that involve a number of discussions of global ocean geochemical changes across the time interval that witnessed Cambrian explosion of metazoans.

New occurrence of Cambroclavus absonus from the lowermost Cambrian of North China and its stratigraphical importance

Li, L.-Y., Zhang, X.-L., Yun, H. & Li, G.-X., October 2015. New occurrence of Cambroclavus absonus from the lowermost Cambrian of North China and its stratigraphical importance. Alcheringa 40, xxx–xxx. ISSN 0311-5518.The problematic Small Shelly Fossil Cambroclavus absonus is described from the Xinji Formation in the Longxian area, which is located near the southwestern margin of the North China Platform. The Xinji Formation, the basal rock unit of the Cambrian in the studied area, yields an assemblage of skeletal fossils that share many common elements with contemporary faunas from South Australia. Sclerites of C. absonus reported herein represent the first occurrence of the species outside Australia, thus extending the palaeogeographic range of the taxon to northern China. To date, palaeogeographic occurrences of Cambroclavus sclerites are restricted to the Peri-Gondwana realm, including South China, Australia, Tarim, Kazakhstan, North China and Western Europe. These occurrences are divided into a Southern Group realm and Northern Group realm. Stratigraphically, Cambroclavus occurs mostly in Cambrian Stage 3 and has three occurrences in Stage 5, separated by Stage 4 in which Cambroclavus has not yet been found. The first appearance datum of Cambroclavus in Cambrian Stage 3 is of importance for regional and inter-regional correlations. In particular, the presence of Cambroclavus absonus in North China allows species-level correlation between North China and South Australia.Luoyang Li [lly@stumail.nwu.edu.cn], Xingliang Zhang [xzhang69@nwu.edu.cn], Hao Yun [yunhao@stumail.nwu.edu.cn], Early Life Institute and State Key Laboratory Of Continental Dynamics, Department of Geology, Northwest University, Xian 710069, PR China; Guoxiang Li [gxli@nigpas.ac.cn], Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, PR China.

New reconstruction of the Wiwaxia scleritome, with data from Chengjiang juveniles.

Wiwaxiids are a problematic group of scale-covered lophotrochozoans known from Cambrian Stages 3–5. Their imbricating dorsal scleritome of leaf-like scales has prompted comparison with various annelids and molluscs, and has been used as a template to reconstruct the articulation pattern of isolated Small Shelly Fossils. The first articulated specimens of Wiwaxia from the Cambrian Stage 3 Chengjiang Konservat-Lagerstätte show that the Wiwaxia scleritome comprised nine equivalent transverse rows associated with outgrowths of soft tissue, but did not possess a separate zone of anterior sclerites. This serial construction is fundamentally incompatible with the circumferential disposition of sclerites in early molluscs, but does closely resemble the armature of certain annelids. A deep homology with the annelid scleritome must be reconciled with Wiwaxia’s mollusc-like mouthparts and foot; together these point to a deep phylogenetic position, close to the common ancestor of annelids and molluscs.

Provenance and temporal constraints of the Early Cambrian Maotianshan Shale, Yunnan Province, China

The Cambrian Maotianshan Shale in Yunnan Province, China contains the well-preserved soft-body fossils of the Chengjiang Biota. The high quality preservation of the non-mineralizing biota (soft tissues and whole carcasses) shows regional and temporal differences, suggesting that paleogeography and local environmental conditions might have contributed to the taphonomy of these fossils. In this paper we present new results from petrographic, geochemical and detrital zircon analyses, and provide a new interpretation about the provenance of the Maotianshan Shale, as well as add to the understanding of the paleogeography of the South China Block during the Cambrian Stage 3. Results from petrographic analysis indicate that the provenance of the Maotianshan Shale is a recycled orogen overall, bordering the western and southwestern margin on the Yangtze Block. The most likely source of the terrigenous material is an exhumed area extending from the Kangdian paleoland to the southeast, paralleling the Song Ma fault zone. Minor regional differences in geochemical and petrographic proxies between the northwestern Jianshan/Ercai area and the southeastern Maotianshan/Xiaolantian area suggest influence of local sources. Sediments of the southeastern province are less mature and samples include minor elements commonly associated with mafic sources. Sediments from the northwestern province are more mature, largely lack mafic components and are enriched in Zr and Hf. The major population of the Maotianshan Shale detrital zircons group at ~800Ma. This crystallization age matches well with the age of a widely spread felsic volcanic and intrusive event associated with the Neoproterozoic Kangdian rift, suggesting that these igneous rocks are most likely a major provenance for the Maotianshan sediments. The youngest zircon population yields consistent Concordia ages of ~520Ma, representing a maximum age constraint on the timing of deposition of the Maotianshan Shale. The zircon crystals of the ~520Ma populations are euhedral with magmatic zoning, indicative of short-distance transport. Volcanic activity along the Song Ma suture zone is a potential source for the ~520Ma detrital zircon suite.

The earliest Phanerozoic carbonate hardground (Cambrian Stage 5, Series 3): Implications to the paleoseawater chemistry and early adaptation of hardground fauna

Carbonate hardgrounds are lithified seafloors formed by synsedimentary cementation of carbonate sediments, which dominantly occur during the period of calcite seas. The earliest typical Phanerozoic hardground known until now was reported from the Furongian of USA, which was suggested to indicate onset of the early Paleozoic calcite sea period. In this study, we report hardgrounds from the early and middle parts of the Cambrian Series 3 (Stage 5 and Drumian) of the North China Platform, which predate previously reported hardgrounds. The hardground surfaces developed on oolitic grainstone, oncolitic wackestone, and microbialite (thrombolite and dendrolite), which sharply truncate the underlying deposits. The radial fibrous calcite cements between the carbonate grains below the hardground surfaces indicate that the cements formed by early marine cementation. EPMA analysis reveals that the fibrous cements typically consist of low-Mg calcite. The hardgrounds are sometimes encrusted by microbialites and coated by hematite, suggesting long exposure to the open seawater after formation of the surface. In addition, detailed review on the sedimentological studies of Cambrian Series 3 to Furongian deposits throughout the world reveals that there may be several other hardgrounds during these times, which could have been overlooked. The abundant occurrence of hardgrounds in Cambrian Series 3 deposits suggests that the general paleoseawater chemistry was suitable to induce synsedimentary cementation of low-Mg calcite, implying that seawater chemistry would have changed from the aragonite to calcite seas during the Cambrian Series 3 or even earlier period. Metazoan encrustors and macroborers possibly could not have adapted to the newly appeared substrate condition yet, until the latest Cambrian Series 3.

Archaeocyathan zonation of the Yangtze Platform: Implications for regional and global correlation of lower Cambrian stages

AbstractDetailed analysis of the distribution of archaeocyaths in five lower Cambrian sections in South China has resulted in the erection of four new archaeocyathan zones and one new archaeocyathan bed. Listed in order of ascending age, these are: theDailycyathus xiuqiensisZone; theDictyocyathus shaanxiensisZone; theSpirillicyathus duplexRange Zone; theSibirecyathus meitanensisRange Zone; and theArchaeocyathus yanjiaoensisbeds. These new subdivisions permit a correlation of lower Cambrian strata both within the area of the Yangtze Platform (South China) as well as between this region and Siberia, Australia, Western Europe and North America. Within the Yangtze Platform area, archaeocyaths of theDailycyathus xiuqiensisandDictyocyathus shaanxiensiszones co-occur with trilobites of the middle and upperEoredlichiaTrilobite Zone of the Qiongzhusian Stage. TheSpirillicyathus duplexand theSibirecyathus meitanensisRange Zone correspond to theMalungia, Yiliangella–YunnanaspisandDrepanuroidesTrilobite zones of the basal and middle Canglangpuan Stage, respectively. Finally, theArchaeocyathus yanjiaoensisbeds belong to thePalaeolenus fengyangensisTrilobite Zone of the upper Canglangpuan Stage. Global correlations based on archaeocyath assemblage zones suggest that: (1) the middle–upper Qiongzhusian of South China correlates with the middle Atdabanian Stage of the Siberian Platform, theWarriootacyathus wilkawillinensisArchaeocyath Zone in South Australia, the middle Issendalenian Stage of Morocco and the lower Ovetian Stage in Spain; (2) the lowermost Canglangpuan Stage of South China correlates with the uppermost Atdabanian of the Siberian Platform, theSpirillicyathus tenuis – Jugalicyathus tardusarchaeocyath zones in South Australia, the uppermost Issendalenian – lower Banian stages of Morocco, the middle Ovetian Stage of Spain and the middle Montezuman Stage of North America; (3) the middle Canglangpuan Stage approximates the lowermost Botoman Stage of the Siberian Platform, middle Banian Stage of Morocco, the uppermost Ovetian – lowermost Marianian stages of Spain and the upper Montezuman Stage of North America; (4) the uppermost Canglangpuan Stage is equivalent of the middle Toyonian Stage of the Siberian Platform,Archaeocyathus abacusbeds in Australia, the middle Bilbilian Stage in Spain and the middle Dyeran Stage (Bolbolenellus eurypariaTrilobite Zone) in North America.

Marine redox variations and nitrogen cycle of the early Cambrian southern margin of the Yangtze Platform, South China: Evidence from nitrogen and organic carbon isotopes

The early Cambrian is a crucial interval, during which the marine environment encountered significant perturbations alongside key biological innovations. The nitrogen isotopic composition of ancient sediments has the potential to record co-variation between the marine ecosystem and environmental conditions. Here we report bulk nitrogen and organic carbon isotopic compositions accompanied by Mo concentration data (a proxy for redox conditions) for a shelf marginal drill core section (Sancha) and a deep basinal outcrop section (Yuanjia) in Hunan province, South China.For the Cambrian Fortunian Stage, variable δ15Nbulk values (−4 to +2‰) and low δ13Corg values (−35‰ to −33‰) from the Yuanjia section indicate that anoxic waters shoaled into the photic zone, with ammonia assimilation competing with anammox in the basinal area. In the late Cambrian Stage 2, low δ15Nbulk and δ13Corg values, together with high Mobulk concentrations (δ15Nbulk=−5 to 0‰; δ13Corg=−33.5 to −31.0‰; >100ppm Mobulk in Sancha section and δ15Nbulk=∼0‰; δ13Corg=∼−33‰ >100ppm Mobulk in Yuanjia section), reflect intense N2-fixation accompanied by intermittent ammonia assimilation under expanded euxinic condition with shoaling chemocline possibly into the photic zone. In the early Cambrian Stage 3, relatively high δ15Nbulk values, alongside positive δ13Corg excursions, at both sections indicate an increasing contribution from 15N-enriched nitrate assimilation and hence more oxic conditions. Shelf margin sediments from the middle and late parts of Cambrian Stage 3 are characterized by low δ15Nbulk values (∼0‰), being lowest in the basinal area (−3 to 0‰). In combination with high Mobulk concentration (>100ppm) and high Mo/TOCbulk ratios (averaging ∼18) in deep basinal sediments, these data suggest that the photic zone became anoxic again, but that sulfidic conditions occurred in the basinal area only. Compared with Stage 2, euxinia was less extensive during this interval, with photosynthetic biomass dominating over chemoautotrophic biomass at both localities.Based on a combination of δ15N data from this study and published data from studies on contemporaneous sections in Yunnan Province and the Three Gorges area, it is possible to reconstruct the spatial variability and temporal evolution of redox conditions in the various marine environments along the southern margin of the Yangtze platform. The basinal area appears to have been dominated by anoxia and intermittent euxinia in the photic zone during most of the early Cambrian (Fortunian Stage to Stage 3) with the exception of more oxic conditions during early Stage 3. On the platform, oxygenated shallow water prevailed with the exception of the notable Cambrian Stage 2 anoxic event. The outer shelf area experienced anoxia from Stage 2 to Stage 3 of the early Cambrian, although of possibly limited areal extent. The high Mobulk concentrations and Mo/TOCbulk ratios suggest that the South China basin was well connected with the considerably oxygenated open ocean and that the mean low δ15Nbulk values (∼+1.6‰) from the basin may reflect generally lower δ15N composition for seawater nitrate in the early Cambrian compared with modern value.

A blind trilobite with Baltic affinities from Cambrian Series 3 of the Iberian Chains, Spain, and its stratigraphical and palaeobiogeographical significance

Three cranidia of a conocoryphid trilobite are described from the lower Caesaraugustan Regional Stage (upper part of provisional Cambrian Stage 5 within Cambrian Series 3) in the Iberian Chains, Spain. The material is from the upper part of the Mansilla Formation and the base of the overlying Murero Formation. It closely resembles Bailiella tenuicincta (Linnarsson, 1879), which has previously been described from the Ptychagnostus gibbus (Cambrian Stage 5) and lower P. atavus zones (lower Drumian Stage) of Baltica (Sweden and Bornholm), and from the P. atavus Zone of West Avalonia (Newfoundland). The record of B. cf. tenuicincta from the Mediterranean margin of West Gondwana provides additional evidence for considering the Acado-Baltic region a discrete biogeographical province, at least during the Cambrian 5 and Drumian ages.